I'll also have to look into Second Home/Vacation Home requirements since this is the first time I'm hearing about it. 

Follow all safety requirements your Insurance Company plus city and county require.This usually includes childproof access and alarms.

You are a real estate professional because you meet the requirements of Section 469(c)(7).

There are legal requirements for disposing personal property left behind by a tenant.

There are a lot of ways to invest in RE and each requires building necessary skills.  

Given interest rate changes, property values, and lender requirements, I’m curious about:How were your acquisition and rehab costs compared to your ARV?

even if its interest only, id want to pay down principal here and there. seems like sellers would offer this interest only route so they have 0 capital gains tax, but id rather pay down a bit of principal in 7yrs. i see its not required but it should allow for principal paydown in promissary note3) any general advice to look out for on seller financing?

You can use your HELOC for down payment to acquire the property instead. paying all cash for this property so no DP required.

- Usually a 5 unit with a boiler means that all the units share the boiler heating system.Which specific utilities in which units are paid by tenants vs shared?

Most states require 27+ days of continuous stay before they are considered a tenant and not a guest.